Portable scales for use in measuring large loads such as heavy vehicles require that they be relatively lightweight with a small size and a low profile. These types of scales are especially useful by state agencies "in the field" to determine if commercial vehicles are operating within allowable load limits. To accomplish the weight measurement, a number of scales are normally placed on the roadway or the shoulder thereof and the vehicle driven thereon such that all of the vehicle's wheels are supported. The environment in which this weight measurement is performed requires reasonably stable temperature characteristics in the scales to provide a reasonably accurate measurement. Normally, prior art scales have provided some type of zeroing mechanism to compensate for temperature variations after the scales have stabilized at an ambient temperature. However, these mechanisms require that the scales be allowed to stabilize prior to performing the weight measurement. These types of scales are impractial in a normal working enviroment since the scaler may be removed from one temperature medium and suddenly placed in another temperature medium, such as a concrete road, for a very short time before making the weight measurement, resulting in substantial inaccuracies.
Previously developed portable scales have utilized some arrangement of springs or hydraulics to measure vehicle weight and still retain some level of portability. In the past, such apparatus has often been bulky in order to achieve the required degree of accuracy. One device that has provided a relatively small scale is disclosed in U.S. Pat. No. 4,007,800 issued to W. Janach, et al. This scale utilizes a plurality of parallel elliptically shaped hollow spring elements. The spring elements all communicate with a gauge and are filled with a fluid. A plate disposed on top of the spring elements compresses the spring elements when subjected to an external weight. The compression of these spring elements results in a volumetric displacement thereby forcing the fluid therein to be displaced into an external reservoir, which is comprised of an expanding bellows-type gauge. The deflection of the bellows is proportional to the deflection of a needle on the gauge. By measuring the volumetric change, the elastic deformation of the spring elements can be converted to a weight measurement.
Although the Janach patent discloses a low profile scale, the use of the elliptical spring elements presents manufacturing and cost problems. This is due to the fact that the elliptical spring elements are essentially tubular members that must be carefully dimensioned and attached to a base on the exterior thereof and then the interiors thereof must be attached to a common channel connecting them to the gauge. In addition, the temperature stability of such previously developed scales has not been completely satisfactory. There thus exists a need for a lightweight portable scale with improved accuracy and with temperature compensation that is relatively simple to manufacture.